Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Sep. 30, 2017		Dec. 31, 2016		Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 67,593		$ 73,875		$ 77,625
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	48,128		54,216
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,442		10,890		10,958
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	13,328		14,080		14,057	[1]
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,950	[2],[3]	23,481	[2],[3]	25,559	[1]
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,408	[2]	5,765	[2]	$ 7,458
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,680		5,738
Commercial Loans [Member] | Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	567		598
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,201		1,458
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,901		3,675
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11		7
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,608		1,941
Commercial Loans [Member] | Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	502		445
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	558		597
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	537		899
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11		0
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,858		3,439
Commercial Loans [Member] | Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	58		152
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	594		803
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,206		2,478
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0		6
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	214		358
Commercial Loans [Member] | Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7		1
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	49		58
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	158		298
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0		$ 1

[1] During the fourth quarter of 2016, we transferred certain client relationships from Commercial Banking to Global Banking and Markets as discussed further in Note 14, "Business Segments." As a result, we reclassified $3.6 billion of loans and $21 million of allowance for credit losses from business and corporate banking to global banking at September 30, 2016 to conform with the current year presentation

[2] During the first quarter of 2017, in conjunction with the creation of the new Corporate Center segment as discussed further in Note 14, "Business Segments," we reclassified loans to HSBC affiliates from global banking to other commercial and revised the prior period to conform with the current year presentation. As a result, other commercial includes loans to HSBC affiliates which totaled $1,820 million and $3,274 million at September 30, 2017 and December 31, 2016, respectively. All tables below have been restated to reflect this reclassification, as applicable. See Note 13, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.

[3] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.